Optical data fiber transmission (e.g. high speed data bus, high speed computer interconnect, local area networks (LAN), etc. has been limited by the availability of a fail-safe multiport optical coupler/repeater. Presently available LAN/data bus concepts have considered various optical energy distribution devices which can be categorized as: (a) passive power splitters (such as fused/biconical couplers) and (b) star couplers (reflective or transmissive) and (c) active repeaters. The devices falling into categories (a) and (b) have the advantage of being completely passive, however, the state-of-the-art of available power launched into an optical fiber, receiver sensitivity and dynamic range and connector, coupler and fiber loss limits the utility of these devices for multiterminal Local Area Networks.
The devices in category (c), active repeaters, lend themselves to high speed multichannel (N greater than 16) linear or ring network topologies, but the integrity of the network is limited by a single point failure in the fiber or a component or power supply in the active repeater.
A fail-safe switch in the data bus terminal has been used to overcome the last-mentioned problem. Such fail-switches have been mechanically or electro-optically actuated switches, and have the drawback of complicating the timing, the synchronization schemes, and/or the signal-to-noise ratio of the overall network. Furthermore, these switches may limit data bus or pipeline speeds as they are slow. Still further, mechanical switches or active repeaters may have a tendency to fail in an "on" mode, thereby swamping a downstream terminal, and are somewhat susceptible to vibration and other such mechanical interference. This last drawback may limit the acceptability of such switches in applications, such as military, where such failures may be totally unacceptable.
Presently, available fail-safe switches cannot accommodate faults, such as a failure in an upstream port in a manner which is both rapid enough for modern technology and which is also rapid and energy-efficient. The mechanical devices are not fast enough for modern applications, and still are susceptible to failure inducing conditions as above discussed. While electro-optical devices may be faster than the mechanical devices, these switches suffer the drawback that they severely limit the distance between stations because they are quite lossy. Often, such switches are active and thus require power for operation, thereby presenting several drawbacks.
A further problem with presently available fail-safe switches used in such systems is that they are not capable of accommadating the "stuck-on" condition of an upstream terminal. As mentioned above, the mechanical switches are susceptible to failure in the "stuck-on" condition. Thus should an upstream terminal fail in the stuck-on mode, presently available fail-safe switches may not be able to accommodate such a condition since many of these switches are set ip to determine only if the data being sent to a terminal is "good" and not if there is a swamping condition present in which to much "good" data is being sent to the terminal.
Some presently available terminals have attempted to overcome these problems by providing a fault-detection system within the terminal that samples the signal from one of several redundant receiver/transmitter units within that terminal and then uses the most acceptable signal. Such fault detection systems, while serving to overcome the signal problem at each terminal are often active thereby requiring additional backup power and may also be expensive. Still further, such fault-detection systems make no provision for a total failure of the particular terminal. That is, if the terminal suffers a total failure, all downstream terminals are affected.
Therefore, there is need of a fault-tolerant coupler/repeater for use in high speed optical fiber data transmission systems which is fast enough and which is reliable enough for modern needs, yet which is not unduly lossy or expensive in nature.